1. Field of the Invention
The present invention relates to a grinding machine with devices for truing and dressing a grinding wheel made of hard material, such as cubic boron nitride (CBN).
2. Description of the Prior Art
For the purpose of truing and dressing such CBN grinding wheels, there has recently been developed a truing and dressing device which, after truing a CBN wheel with a truing roll, having diamond particles secured to its circumferential surface for reforming the wheel surface, dresses the trued CBN wheel by supplying free abrasive grain between the same and a counter roll so as to cut away bond material and to thereby protrude the wheel abrasive grain from the remaining bond material of the wheel surface. Using such a truing and dressing device, surface roughness of the grinding surface of the CBN wheel trued and dressed by the device relatively quickly deteriorates in proportion to the increase in the number of workpieces being ground, as indicated by the dotted line (A) of FIG. 5, and this raises one of the problems of the prior art, namely that the wheel life, or the truing and dressing interval of the CBN wheel, is short.
In order to solve such a problem, the present inventors made various experiments, from which it was found that the free abrasive grain in dressing, when cutting away the bond material of the wheel surface to cause the wheel abrasive grain to protrude somewhat, acted also on the wheel abrasive grain which had been trued, to damage or lift the same from the remaining bond material. From the experiments, it was further found that, in cases where the CBN wheel was re-trued after truing and dressing, a considerably long life could be expected, as indicated by the solid line (B) of FIG. 5.